1. Field of the Invention
This invention relates to position location systems that utilize wireless signals to determine the location of a device.
2. Description of Related Art
Existing position location technologies based on GPS use a network of satellites in earth orbit that transmit signals at a known time. A GPS receiver on the ground measures the time of arrival of the signals from each satellite in the sky it can “see”. The time of arrival of the signal along with the exact location of the satellites and the exact time the signal was transmitted from each satellite is used to triangulate the position of the GPS receiver. A GPS receiver requires four satellites to make a triangulation and the performance of the resulting position location increases as the number of satellites that can be detected increases.
One problem with GPS arises if only three (or less) satellites can be found, and in such an instance it is not possible to accurately locate the GPS receiver. For example, if the GPS receiver's view of the sky is obstructed (e.g. near a tall building) it may not be possible to locate enough GPS satellites to determine location.
Another problem with GPS relates to the amount of time the GPS receiver requires to scan the sky to locate all available satellites (a “cold start”). GPS signals from satellites are highly directional and inherently weak, and therefore finding all available satellites may consume several minutes, even in an open space. Once the satellites have been located by the GPS receiver, only then can they be easily tracked and quickly relocated by the receiver in order to update position in real time.
It has been suggested to use the existing network of cellular base stations to locate position, in a similar manner as GPS. Theoretically-speaking, the exact location of each base station, the exact time at which the base station is transmitting, and the time of arrival of the base station's signal at a mobile station (e.g. cell phone) can be used to triangulate the position of the mobile station. This technique is referred to as Advanced Forward Link Trilateration (“AFLT”). A critical problem faced by the mobile station is to measure the time of arrival of the signals it is receiving from each base station. The simplest method of doing this would be to make a single measurement of the time of arrival for each signal. In one example a single measurement consists of correlating the received signal with a locally generated copy of the transmitted signal, and searching for the peak of this correlation. The goal is to measure the time of arrival of the earliest arrival path from the base station.
In practice, it has proven difficult to implement an AFLT system that can accurately determine the position of a mobile station. Measuring the time of arrival, which is critical to the AFLT process, is difficult in a non-line of sight and/or a dynamic fading environment where multiple paths from the same transmitter are fading in and out unpredictably. For example, if the mobile station is behind an obstruction, the signal from the base station may reflect once, twice, or more along multiple paths before being received by the mobile station. The signal may also go directly through the building, but it may be received as a very weak signal compared to the stronger reflected signal(s).
In part due to the relatively long distance between the satellites and the GPS receiver, a GPS system is not intended to operate in dynamic fading and/or non-line of sight environments. Typically, a GPS receiver makes a single measurement of each satellite, or sometimes it may make multiple measurements with dynamic integration lengths, to determine the correct integration parameters for centering the dynamic range of the available fixed point processor around the received signal strength. Such an approach is not suitable in an AFLT environment, where multiple paths from the same transmitter are fading in and out unpredictably.
It would be advantageous to utilize cellular base stations for position location purposes. The FCC has mandated that cellular operators provide a system for accurately locating the position of cell phones, for reasons including emergency assistance related to “911” calls. For this and other reasons, an effective AFLT system would be useful.